If you have learned about the internals of Git, Merkle Directed Acyclic Graph (DAG) shouldn't be too foreign. As a version control system software, Git is required to keep many versions of a file and distribute them easily to other people. It also needs to be able to check the integrity of the file very quickly.
There are two words that make up Merkle DAG: Merkle and DAG. Let's discuss Merkle first. Actually, the full word of Merkle in this context is Merkle tree. A Merkle tree is a fast way to check whether partial data has been tampered with or not.
